Temperature and heat
* Heat is defined as a form of energy that flows from hotter body to a colder body when they are in thermal contact.
* When two bodies are in thermal contact and there is no net exchange of heat, then they are said to be in thermal equilibrium.
* The physical quantity that is the same for the systems in thermal equilibrium is temperature.
* The SI unit of heat is “joule” and temperature is “Kelvin”.
Ideal gas equation and absolute temperature
* Gas thermometers are used to get accurate temperature readings of a body.
* Boyle’s law: if the temperature of a given mass of gas is constant, then its pressure is inversely proportional to its volume.
* Charles; law: At constant pressure, the volume of a given mass of gas is directly proportional to its temperature, T.
* An ideal gas equation is: PV=µRT
* The relation between the absolute temperature and Celsius temperature is expressed as: T =tc+ 273.15
Thermal expansion
* An increase in the dimensions of a body due to an increase in its temperature is called thermal expansion.
* Expansion along the length of a body while heating is defined as linear expansion.
* Expansion of the surface area of a solid while heating is called volume or cubical expansion.
* The stress developed in a body when not allowing it to expand while heating is called thermal stress.
Specific heat capacity
* The quantity of heat absorbed or emitted, “∆Q”, by a body is directly proportional to its mass and change in temperature, “∆T”.
* Heat capacity of a body is defined as the heat absorbed or emitted per unit change in its temperature.
* The SI unit of heat capacity is J/K.
* Specific heat capacity ‘S’ is the quantity of heat absorbed or emitted by a unit mass of a substance for a change in its temperature of one unit.
* The SI unit of specific heat capacity is J/Kg K.
* When a given substance is expressed in “moles”, its specific heat capacity is known as molar specific heat capacity ‘C’.
* The SI unit of molar specific heat capacity is J/mol K.
Calorimetry
* Calorimetry is the science of measuring heat.
* Principle of calorimetry: when two or more bodies at different temperatures are brought into thermal contact, and if no heat is allowed to escape to the surroundings, then the total heat lost by the hot bodies must be equal to the total heat gained by the cold bodies.
* The amount of heat lost or gained by a body is expressed by the equation: ∆Q=ms∆T
Change of state
* Transition of matter from one state to another is called change of state.
* The change of state from solid to liquid is called melting.
* The change of state from liquid to solid is called fusion.
* The change of state from liquid to gaseous state is called boiling or vaporization.
* The change of state from gaseous state to liquid is called condensation.
* The temperature at which the solid and liquid states of a substance are in thermal equilibrium with each other is called the melting point.
Conduction
* Transfer of heat takes place through conduction, convection and radiation.
* Thermal conduction is a mechanism of heat transfer between two adjacent parts of body or between two bodies in thermal contact.
* Thermal conductivity depends on the state and nature of the substance.
* Change in temperature per unit length is defined as the temperature gradient.
* The SI unit of thermal conductivity Js-1 m-1 K-1 or Wm-1 K-1.
Convection and radiation
* Heat transfer by actual motion of matter from one region to another is known as thermal convection.
* Convection is possible only in fluids.
* Thermal convection can be either natural or forced.
* The steady surface winds blowing from the north east and the south east, towards the equator are called the trade winds.
* The electromagnetic waves emitted by a body by virtue of its temperature is called thermal radiation.
* Heat energy received from the sun is in the form of radiation.
* Heat is transmitted through vacuum in the form of electromagnetic waves.
* Black bodies have a greater capacity of absorbing radiant energy.
Newton’s law of cooling
* The heat lost by the body to its surroundings per unit time is called the rate of cooling of the body.
* The rate of loss of heat of a body is directly proportional to the difference in temperature between the body and its surroundings.
* When the heat transfer is due to a combined effect of conduction, convection and radiation, the rate of cooling is proportional to the difference in temperature.